This invention relates to optical fiber cables with means for preventing flooding of the cable due to unwanted water incursion. More specifically, it relates to cable water blocking techniques using superabsorbent polymer water blocking materials.
Unwanted incursion of water into optical fiber cable sheaths is a well known problem in the cable industry. Changes in ambient conditions may lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of water.
Water also may enter the cable because of damage to the cable. For example, rodent attacks or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter. If not controlled, the water is free to move longitudinally along the cable into splice closures, or terminal equipment, resulting in potential interruptions in service.
Past solutions for unwanted water incursions in optical fiber cable have included filling the void spaces in the manufactured cable with so-called cable plugging compounds. These materials are typically petroleum based compositions that, while relatively effective for controlling cable flooding, are very inconvenient for cable repair and splicing. They require cleaning the petroleum material from fibers prior to splicing, and from equipment and personnel. Extensive efforts in the fiber cable industry have been directed to the development of xe2x80x9cdryxe2x80x9d cable plugging materials. The most widely used of these are tapes or yarns impregnated with superabsorbent polymer materials.
U.S. Pat. No. 5,179,611 discloses a slotted rod type fiber optic cable that includes a water absorptive tape disposed between a jacket section and the slotted rod. The tape is a non-woven fabric, or other sheet-like material, which is typically coated or impregnated with a combination of a thermoplastic elastomer binding agent and a water absorptive resin combined with a water soluble resin. The optical fiber cable industry has established standards for cable performance under flooding conditions. For example, the Telcordia Technologies GR-20 Generic Requirements for Optical Fiber and Cable Industry standard requires that there be no transmission of water under a pressure head of one meter in twenty four hours through a one meter length of cable.
U.S. Pat. No. 4,867,526 discloses a cable having water blocking provisions wherein a tape is interposed between the cable core and jacket. The tape is a non-metallic, non-woven, web-like material impregnated with a so-called superabsorbent material. When exposed to water, the superabsorbent material swells dramatically to block passage of water along the cable.
U.S. Pat. No. 4,909,592 discloses improved impregnated tapes, which, in general, have been used widely and successfully for controlling cable flooding. The details in this patent are incorporated herein by reference. While not disclosed specifically therein, the superabsorbent polymer materials currently used in the industry are fine particulates.
In spite of impressive advancements in cable plugging technology for optical fiber cables, there remains a need for effective and inexpensive water blocking approaches. There is a special need for a water blocking means that is compact, and that operates with a minimum negative impact on microbending losses of the fibers in the cable.
We have discovered that the particle size of the particulates used for the superabsorbent materials in the yarns or tapes used for water blocking within the core tube of central core optical fiber cables has a direct effect on the microbending losses of the optical fibers in the cable. With the benefit of that discovery, we have developed a water blocking tape that comprises a fibrous substrate impregnated with a water absorptive, water insoluble, polymer, wherein the polymer is intertwined and interlocked with fibers of the fibrous substrate, and wherein the polymer comprises particulate material chosen to reduce microbending losses in the optical fibers. The particle size of the polymer superabsorbent is reduced substantially below that currently used in practice. We have found that by carefully controlling the particle size of the polymer powders used in the superabsorbent tape, along with other tape parameters, the result is a significant reduction in microbending losses. The use of fine particles for the superabsorbent material also allows tighter control, and an overall improvement, in the physical properties of the tape, mainly the tape thickness and weight. The latter are important parameters in cable design.
It is also recognized that the swell rate of the superabsorbent material is influenced by the particle size. Very fine particles do not contribute significantly to the swell rate. Accordingly, in a preferred embodiment of the invention the particle size distribution is confined to a narrow range that excludes more than 10 weight % of very fine particles, i.e. particles with a nominal diameter of less than 75 microns.